Synopsis: Fixing a Million-Year Clock

A better measure of an iron isotope’s half-life may lead to new ways of dating astrophysical events that unfold over millions of years.

Radioactive iron-60 (${}^{60}$Fe) is produced at the core of large stars and in supernovae, and it has a half-life of roughly a million years, so its abundance can be used to date astrophysical events on a similar time scale. Scientists have, for example, used the small amount of ${}^{60}$Fe deposited in deep-sea crust to trace the history of supernovae near our Solar System, which may have affected Earth’s climate in the past. But the best measures of ${}^{60}$Fe’s half-life—one performed in 1984, the other in 2009—disagree by nearly a factor of $2$. Now, a new experiment settles the discrepancy, enabling more astrophysical studies based on the isotope, such as the monitoring of nucleosynthesis in stars.

To derive the half-life of a long-lived isotope, scientists use samples containing a known number of the nuclei and detect how many of them decay per second. In the case of ${}^{60}$Fe, its decays are monitored by detecting the gamma rays emitted by its daughter nucleus, cobalt-60. But the main uncertainty in earlier experiments has been the initial number of decaying ${}^{60}$Fe nuclei. Working with an iron sample extracted from irradiated copper, Anton Wallner, at the Australian National University, and his colleagues used accelerator mass spectrometry to determine the small concentration of ${}^{60}$Fe isotopes. By comparing this number to the concentration of ${}^{55}$Fe, another rare isotope, they were able to “cancel out” some of the systematic errors that plagued earlier experiments and accurately gauge the ${}^{60}$Fe amount. The half-life they find agrees well with the 2009 value; averaging the two together, Wallner et al. report a value of $2.60$ million years and a $2%$ uncertainty.

This research is published in Physical Review Letters.

–Jessica Thomas

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